(1) Field of the Invention
The present invention relates to detection of dial pulse information and more particularly to a circuit for dial pulse reconstruction for use in a digital private automatic branch exchange to accurately reconstruct a subscriber's dial pulses despite resistive, inductive and capactive distortion of the dial pulses due to the telephone line loop.
(2) Description of the Prior Art
In a telephone system it is necessary that a switching center accurately obtain digits in the form of dial pulses dialed by a subscriber via his telephone station in order to properly connect one subscriber to another and accurately bill a subscriber. The functioning of the switching center depends entirely upon the accurate reception of these dialed pulses. Distortion of the dial pulses occurs due to the length of the lines connecting the subscriber station to the switching center because of the resistance, inductance and capacitance of the line. Detecting and repeating dial pulses has customarily been performed at the central office by relays. The use of relays for this function currently is prohibitive because of the relatively large size for the relays and their high cost.
The use of electronic circuits to replace the relay's function is typified by the following U.S. Pat. No. 3,772,474 issued on Nov. 13, 1973 to O. G. Wisotzky and U.S. Pat. No. 3,879,583 issued on Apr. 22, 1975 to E. F. Rooks.
The Wisotzky patent discloses a dial pulse correcting circuit using two timing circuits which cooperate to provide minimum timing intervals for the make and break intervals of a dial pulse. The output signals of each of the two timing circuits are gated together to form the leading end and trailing edge of the dial pulse. The two timing circuits disclosed therein are connected in cascade fashion so that the output of the first timing circuit initiates the timing function of the second circuit thereby providing the proper minimum pulse with of a dial pulse.
The circuit disclosed above has various inperfections due to the relatively large quantity of components used and the need to adjust the timing functions by component values. In addition, isolation of the output signal from the input is not provided.
In the Rooks patent a circuit is disclosed for receiving signals from a telephone line, continuously generating and sustaining a reference voltage which is a fixed percentage of peak value of the received signal. The circuit provides an output whenever the received signal level exceeds the reference voltage level. No isolation of input and output signal is provided. This circuit is designed to ignore contact bounce by generation of a dial pulse subsequent to the detection of the incoming originating dial pulse. This operating is performed asynchronously and requires many components and hence utilizes a relatively large amount of space and is costly.
Therefore, it is the objective of the present invention to provide a new dial pulse reconstruction circuit for use in a digital PABX to synchronously detect and reconstruct dial pulses transmitted by a subscriber station to a switching center, utilizing state of the art electronic technology thereby providing for the utilization of a relatively small amount of space and rendering a low cost circuit.
A feature of the present invention includes means for dynamically setting the threshold detection level of the circuit by laser trimming of thick film resistance components of the circuit.